Two stage processes for producing tungsten carbide (WC) are known. For example, in U.S. Pat. No. 3,373,097 entitled "Method For Separation Of A Metal-Containing Halide Phase From A Gangue-Containing Silicate Phase and Electrolysis of Halide Phase To Obtain The Metal" by Gomes et al., issued Mar. 12, 1968, a process for producing tungsten carbide is disclosed. The process involves a molten phase separation employing sodium chloride (NaCl) in which the tungsten reports to a less dense upper halide phase while impurity elements such as calcium, manganese and iron are recovered in a denser lower silicate phase. The separation is effected by heating a mixture of halide salts, concentrates of either scheelite (CaWO.sub.4) or wolframite ((Fe,Mn)WO.sub.4), and a slag former such as sodium silicate to 900.degree. C. to 1,100.degree. C. After fifteen minutes to an hour at the elevated temperature, the phase separation is completed and the halide phase is decanted for processing by molten salt electrolysis.
U.S. Pat. No. 4,489,044 entitled, "Formation Of Tungsten Monocarbide From A Molten Tungstate-Halide Phase By Gas Sparging" by Gomes et al., issued Dec. 18, 1984, reissued as Re 32,612 on Feb. 23, 1988, discloses a process for producing tungsten carbide. The process involves the formation of a sodium chloride/sodium tungstate (Na.sub.2 WO.sub.4) phase by molten phase separation, similar to the process described above. The tungsten monocarbide is produced by sparging the melt of sodium chloride and sodium tungstate with a hydrocarbon gas, particularly methane (CH.sub.4) or natural gas. According to the disclosure, other alkali halides can be substituted for sodium chloride.
In May, 1985, Gomes, Raddatz and Caranahan made a presentation at the Third Tungsten Symposium in Madrid, Spain (May 13-17, 1985) regarding a two step technique for producing a granular tungsten carbide powder directly from scheelite or wolframite concentrates. The concentrates were first reacted at 1,050.degree. C. with a sodium chloride/sodium metasilicate (Na.sub.2 SiO.sub.3) melt. The reaction produces two immiscible liquids: a lighter tungstate-halide (NaCl--Na.sub.2 WO.sub.4) phase containing 99 percent of the input tungsten and a denser silicate slag phase containing 90 to 96 percent of the iron, manganese and calcium oxides. After phase separation, the tungstate-halide phase is sparged with methane gas in a second step to yield granular tungsten carbide. The tungsten carbide is recovered from the reactor by decanting excess salt, cooling, water leaching and scraping. See "Preparation of Tungsten Carbide by Gas Sparging Tungstate Melts", Gomes et al., Journal of Metals, December 1985, pps. 29-32.
The processes described above all include an initial slagging operation in which a tungsten concentrate is combined with a siliceous flux and sodium chloride (other halide sources can be substituted). The tungsten compounds contained in the concentrate (e.g., calcium, iron, or manganese tungstates) react with the sodium chloride and the sodium silicate to produce two immiscible phases: a molten salt and a molten silicate slag. The tungsten preferentially reports to the molten salt phase, while the majority of the impurities are rejected to the slag phase. The viscous slag is more dense than the salt and settles to the bottom of the furnace crucible. The salt phase, which chiefly consists of sodium chloride and sodium tungstate, is forwarded to a second stage for processing into tungsten carbide.
A problem with the methods described above is that the lower density tungsten-containing phase also includes a halide salt (e.g., sodium chloride). During subsequent sparging operations, this halide salt volatilizes and deposits within various components of the gas handling system. This accretion of salt eventually leads to downtime in order to clear the obstructions. The sodium chloride also represents an operating cost. Additionally, the sodium chloride is extremely corrosive and its presence increases the cost of the materials due to the need to employ corrosion resistant materials and results in higher operating costs due to the corrosion. Furthermore, the sodium chloride dilutes the sodium tungstate in the sparging operation, effectively reducing the chemical activity of the tungstic oxide (WO.sub.3).
It would be advantageous to provide a method for forming metal carbide (e.g., tungsten carbide) from a metal-containing mineral using a pyrometallurgical process. Additionally, it would be advantageous to form metal (e.g., tungsten) carbide without the need for forming a fused metal-halide salt. It would be advantageous to provide a process in which a majority of the tungsten input to the system is converted to tungsten carbide. It would be advantageous to provide a process in which tungsten carbide can be formed efficiently and economically without a large amount of system downtime.